hlsl: Avoid HLSL_CLASS_OBJECT, part 1.

HLSL_CLASS_OBJECT does not really have much in common, and a lot of code currently uses it assuming that all objects are resources, which is not actually true. Most users of HLSL_CLASS_OBJECT want to check for specific types, and so we can simplify code a fair amount by promoting all object "base types" to top-level classes.

This series is a first step, comprising some cleanup towards that goal.